1. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to a cell search method and apparatus in an asynchronous wireless communication system for outputting a cell search result by comparing energy ratios of detected candidate cells with a reference threshold ratio.
2. Description of the Related Art
Universal Mobile Telecommunication System (UMTS) is an asynchronous wireless communication system. In the UMTS communication system, base stations are identified by cell specific codes. Assuming the UMTS system has 512 cells, i.e., base stations, each base station is assigned a unique cell identity code.
Since it is time-consuming for a User Equipment (UE) to inspect all 512 cell identity codes to search for a cell it belongs to, a normal algorithm is inefficient to adopt for a cell search procedure. For this reason, the 512 base stations are divided into a number of code groups (e.g., 64 code groups) that are identified by a code group number. In this case, each code group includes 8 specific cell codes.
In the UMTS, the cell search procedure is performed in a stepwise manner. The first step of the cell search procedure is to acquire slot synchronization to a cell with a timeslot in which a Primary Synchronization Channel signal is received at a peak power value. The second step of the cell search procedure is to acquire frame synchronization and identify a code group of the cell found in the first step using the Secondary Synchronization Channel. Finally, the third step of the cell search procedure is to determine a cell specific code used by the found cell through symbol-by-symbol correlation over a Primary Common Pilot Channel (P-CPICH) with all codes within the code group identified at the second step.
At each step of the cell search procedure, an absolute threshold value is used such that, only when the energy of the found cell is greater than the threshold value, a result value of the corresponding step is output. The threshold value is determined as an energy level corresponding to a false alarm probability and a detection probability derived from an energy distribution estimated through a simulation or field test. Accordingly, the threshold value for the cell search procedure is set depending on a cell searcher design.
A radio channel is characterized by a time and location varying property. More particularly in a place surrounded by buildings and having high user density, external interferences increase significantly. Accordingly, a conventional cell search method using the absolute threshold value is likely to cause an unstable change of the threshold value in a rapid variation of a radio channel environment, resulting in malfunctioning of a cell search operation.
Also, the conventional cell search method has a drawback in that the threshold value has to be set differently depending on a difference between the synchronous and asynchronous accumulation durations, and between bit designs of cell searchers.
Therefore, a need exists for a method and apparatus for selecting result values of a cell search process that is not influenced by design change of the cell searcher and variation of the communication environment.